This low pass was designed to allow the probe to directly sample the gases of the extended upper-atmosphere.

Saturn's bulk composition is thought to be about 75% hydrogen with the rest being helium (bar some trace components), explains Nicolas Altobelli, the European Space Agency's Cassini project scientist.

"It's expected that the heavier helium is sinking down," he told BBC News. "Saturn radiates more energy than it's absorbing from the Sun, meaning there's gravitational energy which is being lost. And so getting a precise measure of the hydrogen and helium in the upper layers sets a constraint on the overall distribution of the material in the interior."

Cassini will send all the data back to Earth during its next contact on Tuesday.

Dipping down into the atmosphere should create a drag on the spacecraft, requiring Cassini to use its thrusters to maintain a stable flight configuration and stop itself from tumbling. But the mission's scientists think any buffeting effects ought to be manageable.

They are hopeful that when the post-pass analysis is done, the probe will be permitted to go even lower on the remaining four dip-downs before 15 September's goodbye plunge.

Image copyrightNASA/JPL-CALTECHImage captionArtwork: Cassini is running the narrow gap between the top of the planet's atmosphere and the rings

The Cassini mission still has some big outstanding questions about Saturn. One of these relates to the length of a day on the planet.

Researchers know it is roughly 10-and-a-half hours, but they would like a more precise number.

The solution should come by looking for an offset between the magnetic field and the planet's rotation axis, but frustratingly all the probe's observations to date show these two features to be almost perfectly aligned.

"All magnetic field theory as we know it requires an offset," said Linda Spilker, the US space agency's Cassini project scientist.

"To generate a field, you need to keep the currents in the metallic hydrogen layer inside Saturn flowing, and without the offset the thinking is that the field would simply go away.

"What's going on? Is something shielding our ability to see the offset, or do we simply need a new theory? But without the tilt, without being able to see the tiny wobble, we cannot be more precise about the length of a day."

Dr Spilker said the mission team would continue to work on the problem.

Cassini is a joint venture between the US, European and Italian space agencies. They are ending the probe's operations after 20 years because it is running low on fuel and will soon become uncontrollable.

Scientists want to avoid the possibility of a future collision with Saturn's moons Titan and Enceladus, which could conceivably support simple microbial life. And the only way to prevent that is to deliberately drive the probe to destruction in the atmosphere of the giant planet.

Image copyrightNASA/JPL-CALTECHImage captionCassini's final orbits (blue) take the probe closer to the planet than it has ever been before

Quelle: BBC

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Cloudy Waves (False Color)

Clouds on Saturn take on the appearance of strokes from a cosmic brush thanks to the wavy way that fluids interact in Saturn's atmosphere.

Neighboring bands of clouds move at different speeds and directions depending on their latitudes. This generates turbulence where bands meet and leads to the wavy structure along the interfaces. Saturn’s upper atmosphere generates the faint haze seen along the limb of the planet in this image.

This false color view is centered on 46 degrees north latitude on Saturn. The images were taken with the Cassini spacecraft narrow-angle camera on May 18, 2017 using a combination of spectral filters which preferentially admit wavelengths of near-infrared light. The image filter centered at 727 nanometers was used for red in this image; the filter centered at 750 nanometers was used for blue. (The green color channel was simulated using an average of the two filters.)

The view was obtained at a distance of approximately 750,000 miles (1.2 million kilometers) from Saturn. Image scale is about 4 miles (7 kilometers) per pixel.

Quelle: NASA

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Update: 29.08.2017

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NASA releases inside-out view of Saturn’s rings

Credit: NASA/JPL-Caltech/Space Science Institute

Continuing to make discoveries in the final weeks of a historic mission at Saturn, NASA’s Cassini spacecraft has recorded a unique time lapse movie looking out toward the planet’s iconic icy rings.

Cassini captured imagery to assemble the movie during an Aug. 20 swing through the gap between Saturn and its rings. Ground controllers set Cassini’s wide-angle camera to take images in a low-resolution mode — 512 x 512 pixels — to allow the spacecraft to gather more photos in a short span of time, according to NASA.

“The entirety of the main rings can be seen here, but due to the low viewing angle, the rings appear extremely foreshortened,” NASA said in a statement. “The perspective shifts from the sunlit side of the rings to the unlit side, where sunlight filters through.”

The sequence shows Cassini crossing the relatively thin ring plane as it zipped by Saturn at a speed of some 76,000 mph (122,000 kilometers per hour) relative to the planet’s cloud tops.

“On the sunlit side, the grayish C ring looks larger in the foreground because it is closer; beyond it is the bright B ring and slightly less-bright A ring, with the Cassini Division between them,” NASA said. “The F ring is also fairly easy to make out.”

Running low on fuel, Cassini is set to end its nearly 20-year mission Sept. 15 with a guided plunge into Saturn’s atmosphere. The probe will disintegrate as it descends into the deeper layers of the atmosphere, succumbing to increasing aerodynamic pressures.

Composed mostly of blocks and particles of ice, Saturn’s rings are one of the major targets for Cassini’s scientific instruments during the mission’s final months. Scientists hope to learn the mass of the rings, a figure that should yield an estimate of their age and origin.

Saturn itself is also a focus during Cassini’s grand finale.

Measurements of Saturn’s gravity and magnetic fields could help scientists study the planet’s interior structure, and Cassini’s final five passes near the planet are taking the spacecraft into the outermost reaches of the atmosphere, allowing its instruments to take direct samples.

Cassini will beam back atmospheric data in real-time during its final descent into Saturn next month until the probe’s control thrusters are unable to keep the craft’s antenna locked on Earth.

Launched from Cape Canaveral in October 1997, the spacecraft spent much of its time at Saturn making observations of the planet’s numerous moons. Cassini dropped a European Space Agency lander to Titan’s surface for a parachute-assisted landing in 2005, and found evidence that Enceladus harbors a global ocean of water buried beneath a veneer of ice.

NASA officials want to end the mission in a guided manner while the spacecraft is still healthy to avoid the chance an uncontrolled Cassini could crash into Titan or Enceladus, potentially spoiling habitats that might be prime for finding alien microbial life.

Since April, Cassini has made repeated passes between Saturn and its rings. Two more flybys are planned before the final dive Sept. 15.

Quelle: SN

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Update: 30.08.2017

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NASA’s Cassini spacecraft nears a fiery, brutal end, when it will plunge into Saturn

NASA’s Cassini spacecraft has explored the Saturn system since 2004. This video recaps some of the mission's discoveries about the giant planet, its rings and its moons.

After 13 years of observing Saturn, its rings and its myriad moons, NASA’s Cassini spacecraft is less than three weeks away from a fiery, brutal end.

Early in the morning on Sept. 15 the aging spacecraft will hurl itself into Saturn’s atmosphere at speeds of more than 75,000 mph.

It’s a deliberate death plunge from which it has no hope of returning.

Within three minutes of diving into Saturn’s tenuous upper layers, the two-story-tall spacecraft will be torn apart.

In the end, Cassini will become part of the very planet it has studied for more than a decade.

“I like to say it’s going out in a blaze of glory,” said Linda Spilker of the Jet Propulsion Laboratory, the project scientist for the mission. “It will be trailblazing until the very last second.”

Earth-bound astronomers will keep a close eye on the planet at the moment of Cassini’s death to see whether they can detect a small flare as the spacecraft burns up like a meteor in the Saturnian sky.

But they don’t expect to see much.

“The mass of Cassini is so small compared to the mass of the planet,” Spilker said. “And the plunge is happening on Saturn’s day side.”

She added that there is no fear that Cassini’s death dive will contaminate the ringed giant. Molecules from the spacecraft will quickly spread out across the planet, which is big enough to hold more than 700 Earths.

Still, Cassini’s suicide mission is not all gloom and doom. The spacecraft will venture deeper into Saturn’s atmosphere than ever before, collecting brand new data and beaming it back to Earth up until the last seconds of its life.

“We are reconfiguring the spacecraft and turning Cassini into an atmospheric probe,” said Earl Maize, the mission’s program manager, who also works at JPL.

Cassini could make it as far as 120 miles into the Saturnian atmosphere before all its instruments cease to function, mission engineers said.

“I find great comfort that Cassini will continue teaching us until the very last second on Sept. 15,” said Curt Niebur, Cassini program scientist based at NASA’s headquarters in Washington.

The flagship mission launched in 1997 and entered the Saturnian system in 2004.

Over the last 13 years it has discovered plumes of water ice on the moon Enceladus, confirmed the presence of methane and ethane lakes and rivers on Titan, watched the seasons change on Saturn, discovered six new moons and revealed complexities in the planet’s rings that had never been seen before.

“We’ve rewritten the textbooks on Saturn,” Spilker said. “Literally, there are so many new books coming out.”

Although the spacecraft’s instruments continue to work flawlessly, its gas tank is nearing empty. Mission planners decided to crash Cassini into Saturn to avoid any risk of contaminating Enceladus or Titan, two of Saturn’s moons that could harbor the ingredients necessary for life.

Beginning in April, Cassini began a series of orbits that took it speeding through the gap between the planet and its rings for the first time. This has allowed scientists to address new questions including the age of the rings and how quickly the planet’s interior is spinning.

By Sept. 15, the spacecraft will have completed 22 of these orbits to collect new data even as its end looms.

“Who knows what new mysteries the next two weeks will bring?” Spilker said.

Quelle: Los Angeles Times

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Saturn Plunge Nears for Cassini Spacecraft

NASA's Cassini spacecraft is shown heading for the gap between Saturn and its rings during one of 22 such dives of the mission's finale in this illustration. The spacecraft will make a final plunge into the planet's atmosphere on Sept. 15. Credit: NASA/JPL-Caltech

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NASA's Cassini spacecraft is 18 days from its mission-ending dive into the atmosphere of Saturn. Its fateful plunge on Sept. 15 is a foregone conclusion -- an April 22 gravitational kick from Saturn's moon Titan placed the two-and-a-half ton vehicle on its path for impending destruction. Yet several mission milestones have to occur over the coming two-plus weeks to prepare the vehicle for one last burst of trailblazing science.

"The Cassini mission has been packed full of scientific firsts, and our unique planetary revelations will continue to the very end of the mission as Cassini becomes Saturn's first planetary probe, sampling Saturn's atmosphere up until the last second," said Linda Spilker, Cassini project scientist from NASA's Jet Propulsion Laboratory in Pasadena, California. "We'll be sending data in near real time as we rush headlong into the atmosphere -- it's truly a first-of-its-kind event at Saturn."

The spacecraft is expected to lose radio contact with Earth within about one to two minutes after beginning its descent into Saturn's upper atmosphere. But on the way down, before contact is lost, eight of Cassini's 12 science instruments will be operating. In particular, the spacecraft's ion and neutral mass spectrometer (INMS), which will be directly sampling the atmosphere's composition, potentially returning insights into the giant planet's formation and evolution. On the day before the plunge, other Cassini instruments will make detailed, high-resolution observations of Saturn's auroras, temperature, and the vortices at the planet's poles. Cassini's imaging camera will be off during this final descent, having taken a last look at the Saturn system the previous day (Sept. 14).

In its final week, Cassini will pass several milestones en route to its science-rich Saturn plunge. (Times below are predicted and may change slightly; see https://go.nasa.gov/2wbaCBT for updated times.)

Sept. 9 Cassini will make the last of 22 passes between Saturn itself and its rings -- closest approach is 1,044 miles (1,680 kilometers) above the clouds tops.

Sept. 11 -- Cassini will make a distant flyby of Saturn's largest moon, Titan. Even though the spacecraft will be at 73,974 miles (119,049 kilometers) away, the gravitational influence of the moon will slow down the spacecraft slightly as it speeds past. A few days later, instead of passing through the outermost fringes of Saturn's atmosphere, Cassini will dive in too deep to survive the friction and heating.

Sept. 14 -- Cassini's imaging cameras take their last look around the Saturn system, sending back pictures of moons Titan and Enceladus, the hexagon-shaped jet stream around the planet's north pole, and features in the rings.

Sept. 14 (5:45 p.m. EDT / 2:45 p.m. PDT) -- Cassini turns its antenna to point at Earth, begins a communications link that will continue until end of mission, and sends back its final images and other data collected along the way.

Sept. 15 (4:37 a.m. EDT / 1:37 a.m. PDT) -- The "final plunge" begins. The spacecraft starts a 5-minute roll to position INMS for optimal sampling of the atmosphere, transmitting data in near real time from now to end of mission.

Sept. 15 (7:54 a.m. EDT / 4:54 a.m. PDT) -- Cassini's thrusters are at 100 percent of capacity. Atmospheric forces overwhelm the thrusters' capacity to maintain control of the spacecraft's orientation, and the high-gain antenna loses its lock on Earth. At this moment, expected to occur about 940 miles (1,510 kilometers) above Saturn's cloud tops, communication from the spacecraft will cease, and Cassini's mission of exploration will have concluded. The spacecraft will break up like a meteor moments later.

As Cassini completes its 13-year tour of Saturn, its Grand Finale -- which began in April -- and final plunge are just the last beat. Following a four-year primary mission and a two-year extension, NASA approved an ambitious plan to extend Cassini's service by an additional seven years. Called the Cassini Solstice Mission, the extension saw Cassini perform dozens more flybys of Saturn's moons as the spacecraft observed seasonal changes in the atmospheres of Saturn and Titan. From the outset, the planned endgame for the Solstice Mission was to expend all of Cassini's maneuvering propellant exploring, then eventually arriving in the ultra-close Grand Finale orbits, ending with safe disposal of the spacecraft in Saturn's atmosphere.

"The end of Cassini's mission will be a poignant moment, but a fitting and very necessary completion of an astonishing journey," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "The Grand Finale represents the culmination of a seven-year plan to use the spacecraft's remaining resources in the most scientifically productive way possible. By safely disposing of the spacecraft in Saturn's atmosphere, we avoid any possibility Cassini could impact one of Saturn's moons somewhere down the road, keeping them pristine for future exploration."

Since its launch in 1997, the findings of the Cassini mission have revolutionized our understanding of Saturn, its complex rings, the amazing assortment of moons and the planet's dynamic magnetic environment. The most distant planetary orbiter ever launched, Cassini started making astonishing discoveries immediately upon arrival and continues today. Icy jets shoot from the tiny moon Enceladus, providing samples of an underground ocean with evidence of hydrothermal activity. Titan's hydrocarbon lakes and seas are dominated by liquid ethane and methane, and complex pre-biotic chemicals form in the atmosphere and rain to the surface. Three-dimensional structures tower above Saturn's rings, and a giant Saturn storm circled the entire planet for most of a year. Cassini's findings at Saturn have also buttressed scientists' understanding of processes involved in the formation of planets.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.

Quelle: NASA

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Update: 6.09.2017

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Cassini's Last Photos Will Be Spectacular

NASA's Saturn probe has advanced scientists' search for life in the universe.

If you see a space scientist looking forlorn this month, it's probably because NASA's Cassini mission to Saturn is about to end. There's nothing anyone can do about it; the spacecraft has already been kept operational for nine years past its planned expiry date. And now it's out of fuel.

But Cassini will be missed. Since 2004, it's been scrutinizing the most recognizable planet in our solar system -- and many of Saturn's rings and moons. Now the space scientists operating the craft have found a way to make its Grand Finale especially memorable and important, by shifting its orbit to collect extra data, even as it prepares for a fatal dive into Saturn itself.

If space scientists were collectively asked to name the highlights of what Cassini has taught us about Saturn, they might shout in unison, "Titan!" That's Saturn's largest moon and the only body in the solar system outside of Earth that has a thick, nitrogen-dominated atmosphere and liquids that persist on the surface. Titan's liquids are not water, but rather methane, ethane and other hydrocarbons. Nevertheless, this moon might have the right stuff to provide the complex chemistry that's needed to create a suitable environment for life. Titan is so special to space scientists that the Cassini mission included the Huygens Probe, which in 2005 descended through the moon's atmosphere, landed, and sent us the first images of its surface. This is how we learned that Titan also shares with Earth many geologic features, including river drainage systems, lakes and dunes.

After giving Titan its due, scientists would point to Enceladus, another of Saturn's moons, but much smaller -- just 500 kilometers in diameter, with a surface that's mostly solid ice. In 2005, Cassini flew through plumes of vapor emanating from a saltwater ocean beneath the ice near Enceladus's south pole. This liquid ocean is probably in contact with rock below and may receive energy from tides and radioactive elements, which could provide important catalysts for the development of life. Future missions to Enceladus can be designed to look for evidence of life without landing on the surface, by sampling the plumes expelled into space.

These are just two of Cassini's many significant discoveries. The Grand Finale is guaranteed to add more. For years, Cassini had been orbiting close to Saturn's equatorial plane, in order to efficiently investigate the moons. But beginning in late April of this year, the spacecraft shifted course and has been flying on a path more perpendicular to the equator, a north-to-south elliptical route that reaches a point closest to Saturn about once a week. These orbits include dramatic passes between Saturn and its rings that allow Cassini to observe the planet in unprecedented detail. With the completion of each one, the scientific community exhales deeply, relieved that the spacecraft has survived for another round. Twenty of the planned 22 orbits have been completed. The last one, in which the spacecraft is to descend into Saturn's atmosphere and burn up from exposure to the high pressures and temperatures, is set to take place on Friday, Sept. 15.

The Cassini mission has been a constant presence in the careers of many space scientists working today, whether it was under development, traveling to Saturn, or carrying out its extended missions. Sept. 15 will mark the end of an era, and this comes with an immense sense of loss. Yes, there are other fantasticmissions under way in our solar system. But Cassini is special. The data it has provided have advanced our understanding of the solar system, and informed our search for life elsewhere in the universe.

On Sept. 15, I will raise a glass to this wonderful mission and all the scientists and engineers who made it possible. I will say goodbye and thank you to Cassini for inspiring my work and demonstrating what remarkable things humans can accomplish when they work together.

I encourage everyone to learn more about the Cassini mission and how it has revolutionized our thinking of planetary processes. This website is a good place to start: https://saturn.jpl.nasa.gov/. Follow the Grand Finale with the @CassiniSaturn twitter feed.

Quelle: Bloomberg

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Update: 11.09.2017

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Prometheus and the Ghostly F Ring

The thin sliver of Saturn's moon Prometheus lurks near ghostly structures in Saturn's narrow F ring in this view from NASA's Cassini spacecraft. Many of the narrow ring's faint and wispy features result from its gravitational interactions with Prometheus (86 kilometers, or 53 miles across).

Most of the small moon's surface is in darkness due to the viewing geometry here. Cassini was positioned behind Saturn and Prometheus with respect to the sun, looking toward the moon's dark side and just a bit of the moon’s sunlit northern hemisphere.

Also visible here is a distinct difference in brightness between the outermost section of Saturn's A ring (left of center) and rest of the ring, interior to the Keeler Gap (lower left).

This view looks toward the sunlit side of the rings from about 13 degrees above the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 13, 2017.

The view was acquired at a distance of approximately 680,000 miles (1.1 million kilometers) from Saturn. Image scale is 4 miles (6 kilometers) per pixel.

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Saturn-lit Tethys

Cassini gazes across the icy rings of Saturn toward the icy moon Tethys, whose night side is illuminated by Saturnshine, or sunlight reflected by the planet.

Tethys was on the far side of Saturn with respect to Cassini here; an observer looking upward from the moon's surface toward Cassini would see Saturn's illuminated disk filling the sky.

Tethys was brightened by a factor of two in this image to increase its visibility. A sliver of the moon's sunlit northern hemisphere is seen at top. A bright wedge of Saturn's sunlit side is seen at lower left.

This view looks toward the sunlit side of the rings from about 10 degrees above the ring plane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on May 13, 2017.

The view was acquired at a distance of approximately 750,000 miles (1.2 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 140 degrees. Image scale is 43 miles (70 kilometers) per pixel on Saturn. The distance to Tethys was about 930,000 miles (1.5 million kilometers). The image scale on Tethys is about 56 miles (90 kilometers) per pixel.

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So Far from Home

With this view, Cassini captured one of its last looks at Saturn and its main rings from a distance. The Saturn system has been Cassini's home for 13 years, but that journey is nearing its end.

Cassini has been orbiting Saturn for nearly a half of a Saturnian year but that journey is nearing its end. This extended stay has permitted observations of the long-term variability of the planet, moons, rings, and magnetosphere, observations not possible from short, fly-by style missions.

When the spacecraft arrived at Saturn in 2004, the planet's northern hemisphere, seen here at top, was in darkness, just beginning to emerge from winter (see PIA06164). Now at journey's end, the entire north pole is bathed in the continuous sunlight of summer.

Images taken on Oct. 28, 2016 with the wide angle camera using red, green and blue spectral filters were combined to create this color view. This view looks toward the sunlit side of the rings from about 25 degrees above the ringplane.

The view was acquired at a distance of approximately 870,000 miles (1.4 million kilometers) from Saturn. Image scale is 50 miles (80 kilometers) per pixel.